Automatic control of television receiving systems



J. C. WILSON May 6, 1941.

Filed March 15, 1938 4 u m+ m+ n u Ill lwl I-l b fi 0E H h A 3 Maw v anA W on 3 8 55.26 2 553%. v was: m z an or %z m uz m d .o 2.. g 3:...mwmwag rm. z wumfiw m 5. .6 .3523 a m @8580: z 3 G 3053 mew uww amamwi5.3468 655.. 2 nk .1 mi Nu INVENTOR JOHN '0. WILSON W 4 ATTORNEY y 1941-J. c. WILSON 2,240,533

AUTOMATIC CONTROL OF TELEVISION RECEIVING SYSTEMS Filed March 15, 1938 2Sheets-Sheet 2 i A Mm; UE m2. 5; W=Ql;fl=JL-- FIG.3.

Dnffmm Vohogo IN'VENTOR J 0. WILSON ATTORNEY Patented May 6, 1941UNITED," STATES... PATENT OFFICE AUTOMATIC CONTROL OF TELEVISION 1RECEIVmG SYSTEMS John O. Wilson, Bayslde, N.

Hazeltlne Corporatio ware Y., asslgnor to n, a. corporation of Dela-Applicatlon March 15, 1938, Serial No. 195,955

10 Claims. (Cl. 178-43) This invention relates to television system and,more particularly, to the receiving automatic control of one or moreoperating characteristics respond to initiations of successive lines andfields in the scanning of the image.

At the receiver, a beam is so deflected as to scan and illuminate atarget in a series of fields of parallel lines. The synchronizingcomponents of the received signal are utilized to-control the scanningapparatus of the receiver so as to synchronize its operation with thatof similar apparatus utilized at the transmitter in developing thesignal. trolled by the light-modulation components, thereby toreconstrrct the image.

In certain proposed television systems, the video-frequency modulationdeveloped from the image is so impressed on the carrier that increasesin the carrier amplitude correspond to increases in brightness of theimage, this being termed positive modulation; the backgroundillumination is transmitted directly, that is, by variation of theaverage carrier intensity during the The intensity of the beam isconamplification control. More particularly, since the average intensityof the. carrier is varied in accordance with background light-modulationcomponents, neither the average nor the peak carrier intensity can beutilized to eflect satisfactory automatic amplification control in theconventional manner, which requires some controlling effect dependentsolely upon the average trace intervals directly in accordance with thelow-frequency or steady background light variations. Betweentraceperiods,,a blanking signal is transmitted which corresponds in amplitudeto black or blacker than black and establishes a predetermined blankinglevel. The synchronizing pulses are impressed on the carrier duringtheblanking intervals in the opposite sense to the light-modulationsignals, ordinarily periodically reducing the carrier amplitude to zero.

While systems utilizing the type of signal just mentioned have, ingeneral, given satisfactory results, they have been objectionable inthat it has been difficult or impossible heretofore to providesatisfactory automatic control of the system characteristics inaccordance with the average received carrier amplitude and independentof light modulation, as, for example, automatic relatively fixedamplitude,

carrier intensity as determined by the power of the transmittingstation, its distance, fading, etc., and independent of the degree ofmodulation. Nor can the amplitude of the'synchronizing pulses .of such asignal be made readily available for this purpose, as it can in the caseof negative modulation where these pulses are transmitted on outwardpeaks of modulation at 9.

since, in positivemodulation systems, the synchronizing pulses areordinarily transmitted on inward peaks of modulation correspondingsubstantially to .zero amplitude of the carrier wave. .Variousarrangements have, however, been devised heretofore for providing-anautomatic control of an operating characteristic of apositive-modulation television system in accordance with the averageamplitude of received carrier wave and independent of light however,these systems in that they were detheir operation highly critical.

It is an object of the present invention, therefore, to provide atelevision receiver including improved means for automaticallycontrolling one or more operating characteristics of the receiver inaccordance with the average intensity of the received modulated-carriersignals and independently of-the light-modulation components. It is afurther object of the invention to provide an improved automaticamplification control system for television receivers adapted for thereception of positively-modulated television signals includingbackground and high-frequency light-modulation components.

It is a further object of the invention to provide a. system of thecharacter described which operates independently of any timing impulses.

In accordance with the present invention, a television receiver adaptedfor the reception of positively-modulated television signals, includingbackground components, synchro components, and video-frequencylight-modulation components, comprises asynchronous control meansresponsive to a predetermined intermediate amplitude level of themodulation envelope of the signals, and including a time-constantcirarefurther provided for which means may i her. A stabilizer is preferablycoupled to the output circuit of the 1 frequency amplifier .l

components on the output acontrolling effect. Means said synchronizingthereof for developing eifect for adjusting an of the receiver.

In a preferred embodiment of the invention, a television receiver"comprises means for detectingthe signal to derivethe modulationenvelope,

be a conventional diode rectidetecting means for establishing a-fixedlevel for the peaks of the synchronizing pulses and developinga signalcomponent varying in accordance with the blanking level of the fmodulation envelope. Suitable rectifying means, for example, a dioderectifier, is provided fordeveloping a unidirectional control-biasvolt-- age fromthe peak value .of the voltage representing thedifierence between the stabilized de- The unidirecbe utilized to tectedsignal and a fixed voltage. tional voltage thus developed may system,thereby to ma tain the signal-output intensity within arelatively'narrow range for a wide range of signal-inputintensities.

For a better understanding of the present invention, togethenwith otherand further objects cuit for substantially eliminating the effect ofcontrol elements The intensity of of the generators l8 and. I. theelectron beam of the tube I5 is thus modulated or controlled inaccordance utilizing this controllin operating characteristic 'quencygenerators l8 with the video-frequency voltages impressed upon thecontrol electrode of the tube in the usual manner. Saw-tooth current orvoltage-scanning waves are generated in the lineand field-freand I9,which are controlled by synchronizing-voltage impulses supplied fromcontrol the amplification in various stages of the thereof, reference ishad to the following description taken in connection w'th theaccompanying drawings, and its scope will be p0 ted out in the appendedclaims.

In the accompanying drawings, Fig. l is a circuit diagram, partiallyschematic, of a cathoderay tube television receiving system includingcircuits embodying the present invention; Fig. 2 is a diagrammaticillustration of a positively-modulated television carrier wave, to aidin the understanding of the invention, 6 are groups of wave formsrepresenting the derived signal developed at different points in thereceiver of Fig. 1.

Referring now more particularly to the drawings, the system thereillustrated comprises a receiver of the superheterodyne type includingan antenna and ground. system Ill-II connected to a radio-frequencyamplifier l2 to which is connected in cascade,-in the order named, anoscillator-modulator l3, s an intermediate-frequency amplifier l4,a'detector l5, a video-frequency amplifier 16,. and a cathode-raysignalreproducing-tube ll. A line-frequency generator l8 and afield-frequency generator l9 are also coupled to the video' f requencyamplifier and connected to the scanning elements of the cathoderay tubein the-conventional manner. The stages or units Ill-l9, inclusive, mayall be of conventional'well-known'construction so that detailedillustrations and descriptions thereof are deemed unnecessary herein; 11

Referring briefly, however, to the operation of the system describedabove, television signals intercepted by the antenna circuit Ill-ll areselected and amplifiedin the radio-frequency amplifier l2 and suppliedto the oscillator-modulator while Figs. 3', 4, 5, and

signal are derived ,mined intermediate The tube 20 may be of any suitthevideo-frequency amplifier l6, and applied to the scanning elements ofthe cathode-ray tube I! to produce electric scanning fields, thereby todeflect the ray vertically and horizontally so as to trace a rectilinearscanning pattern on the screen of the tube and-thereby reconstruct thetransmitted picture.

Referring now more particularly to the portion of the system of Fig. 1embodying the present invention, for the purpose of developing acontrol-bias voltage there is provided an asynchronous control means'including a vacuum-tube repeater 20 connected fier 2|, and a direct 22,as shown, which current reversing amplifier is responsive to apredeteramplitude' level of the received signal. able type having aninputcircuit, comprising a coupling condenser 23 and nected to theoutput circuit of the detector l5. An output circuit is provided for thetube 20 comprising a resistor 25 and a parallel condenser 26 having apredetermined time constant which is long compared to the duration ofthe line-synchronizing pulses and blanking intervals, but short comparedto the field blanking intervals. For conventional systems utilizing, forexample, frames of 441 lines per second, the resistor 25 may be of theorder of 100,000 ohms and the condenser 26 -of the order of 0.006microfarad, providing a time constant of 0.0006 second. The dioderectifier 2| is coupled across the output circuit of the tube 20 and isprovided with a load circuit comprising a parallel-connected resistor 21and condenser 28 having a large time constant. This time constant shouldbe considerably greater than the field-scanning period, which may, forexample, be %0 second, in which event the time constant may be of theorder of 6 second. The input electrodes of the reversing amplifier areconnected across the resistor 21, while its anode circuit includes aload resistor 29. Operating potentials are applied to the anodes of thetubes 20 and 22 by way of their respective resistors 25 and 29 fromsuitable sources, indicated at +3. The less positive end of the resistor29 is connected, by way of a suitable filter including seriesresistors30 and shunt condensers 3| and conductors 32, to the controlelectrodes of one or more vacuum tubes included in the amplifier l2,oscillator-modulator l3, and amplifier M, as shown. The lead 32 mayinclude a negative-biasing battery 33 to compensate for the highpotential of the anode of the tube 22 from which the volume controlpotential is derived. However, if the to act as a stabilizer, a recti-'leak resistor 24, con-- The lesser amplitude, as might be caused byoccasional fading. In Figs. 3-6, inclusive, corresponding wave portionsare shown, illustrating the formof the signal derived at different partsof the system.

The points of the wave of Fig. 2 at which the carrier amplitude isreduced to zero, certain of which are indicated at L, represent lineretrace periods or synchronizing pulses. The portions between the pulsesL correspond to the trace portions of successive lines and aremodulated, as. indicated at M1 and M2, by the high-frequency andbackground light components; During the frame retrace periods, part ofone of which is indicated at YY, the amplitude of the wave isindependent of light modulation. The, frame retrace periods ordinarilyinclude the line pulses, as well as additional broad pulses, not shown,which latter constitute collectively a frame-synchronizing pulse.

It is well known that, with the positive .type of modulation, such as isrepresented in Fig. 2, increases in light intensity of the image beingscanned are represented by increases in the amplitude of the carrieramplitude and the average intensity of the signal is varied inaccordance with the low-frequency or unidirectionalbackground-illumination components of the image. Thus, in the particularportions of the wave shown, during the line-trace modulation periodsrepresented at M1, the amplitude of the carrier is such that it reachesthe white level, whereas this does not occur during the modulationperiods represented at Me. Since the average intensity of the carrier isthus varied in accordance with background-illumination variations, thesecarrier variations cannot be used for the purpose of ampliflcationcontrol. Nor, as stated above, are there in this type of wave anysynchronizing pulses which could be utilized for the purpose inquestion, since the signal is reduced to zero during the duration ofline-retrace or line-synchronizing pulses. However, the modulationenvelope, inchides a certain representative level, for example, theblanking level, at which the signal may represent black or blacker thanblack and which .appears in the signal not only for black pictures, butalso during the frame retrace periods. This level affords a measure ofthe intensity of the carrier wave which is independent of lightmodulation and of other signal characteristics, such, for example, asits periodicity or synchronization. This level may, therefore; beutilized automatically to control a characteristic of the receiver, suchas its amplification.

In the operation of the present invention, therefore, themodulated-carrier wave is detected by the detector l5 to derive avoltage wave corresponding to the modulation envelope, which voltage isimpressed upon the input electrodes of the stabilizing tube by way'ofthe non-conductive circuit including the grid condenser 23 and leakresistor 24. The grid condenser and leak serve to so vary the bias ofthe control grid, which, in the absence of a signal, is provided withzero bias, that the signal is stabilized; that is, to ensure that thepeaks of the synchronizing pulses are at a substantially fixed level asimpressed on the control grid. Where the connection between the controlcircuit and the detector issubstantially a direct one, so that theunidirectional component of the signal is not lost, the signal may besulficiently stabilized without the use of the stabilizing tube 20.However, where a nonconductive coupling intervenes between the detectorand the control circuit, as in Fig. 1, the unidirectional component issuppressed and the signal tends to center itself about the zero axis andto appear as illustrated by the curves of Fig. 3. In this case,therefore, it must be stabilized as explained.

The characteristics of the tube 20, also, are preferably such as to cutoff or limit at least a part of the video-frequency portion of the wave,so that the wave form of the current in the anode circuit of the tube 20is as illustrated by the wave forms of Fig. 4. Due to the time constantprovided by the anode load circuit comprising resistor 25 and condenser26, which is long compared with the duration of the line-synchronizingpulses and line-blanking intervals, but short compared with thefield-blanking intervals, the voltage developed across the condenser 26is not of the same form as the anode current, but is of the form shownin Fig. 5. In other words, the output circuit time constant is such assubstantially to eliminate the effect of the line-synchronizingcomponents and may be said to cause the system to ignore signal levelslower than that desired, for example, the line-synchronizing pulselevel, because of their short duration, that is, the

, condenser 26 may be considered as by-passing the synchronizing pulsesor components. The portions m, m of the wave forms of Fig. 5 arerepresentative of, and vary with, the signal level during the fieldblanking intervals. However, these portions are minima in the waves ofFig. 5 so that they cannot be measured directly.

In order to measure these voltage minima. there is obtained thedifference voltage between the voltage across condenser 26 shown in Fig.5 and the operating voltage or +3 voltage, which difference voltage isdeveloped across the resistor 25 with wave forms such as shown in Fig.6. It is noted that this difference voltage has amplitude maxima p, pcorresponding to the minima m, m of Fig. 5 and occurring during, anddependent solely upon, the blankinglevel during the field blankingintervals. This difference voltage, therefore, constitutes a controlvoltage which has a peak amplitude corresponding to variations in theaverage intensity ofthe received carrier and independent of lightmodulation, this being indicated at p and p in Fig. 6 for the signals ofdifferent intensities. Moreover, the control signal is derived inresponse to a predetermined level of the modulation envelope occurringfor a predetermined duration greater than the duration of aline-synchronizing pulse, in the present case, the blanking level, andindependently of other characteristics thereofparticularly,independently of pulses dependent upon the periodicity ofsynchronization of the signal. This control signal is thereuponrectified by the peak rectifier or diode 2|, thereby developing acrossits load resistor 21 va control-bias voltage dependent solely upon theportions p and p of the waves of Fig. 5 while the difference voltage isas shown in Fig. 6, since operating potential is applied to the anode ofdiode 2| from the source +B by way of resistor 21, this potential isalso on the control grid of the tube 22. A biasing battery is,therefore, included as shown in the cathode circuit of the tube 22 tocompensate for the +3 voltage. The difference or control-bias voltage isapplied negatively to the grid of tube 22 wherein it is amplified with areversal in polarity. This rectified and amplified control-bias voltageacross the load resistor 29 of tube 22 is fairly steady and isproportional to the average intensity of the carrier and independent oflight-modulation components. The filter comprising the resistors 30 andcondensers 3l serves to remove residual fluctuations from the biasvoltage. The resultant unidirectional-bias voltage is impressednegatively on the control electrodes of one or more of the tubes in thestages l2, l3, and H to control the amplification of these stagesinversely in accordance with variations of the average carrier intensityand independently of light-modulation components. Thus, theoutput-signal intensity of the channel, including the stages l2, l3, andi4, is maintained within a relatively narrow range for a wide range ofreceived signal intensities.

As explained above, the amplifier 22 simply serves to reverse thepolarity of the unidirectional-bias voltage developed by the rectifier2| to develop a voltage which increases negatively with increasingcarrier amplitude and independent of light-modulation components. Thisis, of course, the required polarity for effecting automaticamplification control. Various other embodiments of the invention willbe readily apparent to those skilled in the art, in some of whichembodiments a reversing amplifier will be unnecessary. For example, thetubes and 2| may be so connected that the voltage developed in theoutput circuit of the tube 2! increases negatively with increasingcarrier amplitude, as by including a resistor equivalent to resistor 25in the cathode circuit instead of the anode circuit of tube 20.

It will be noted that where .the expression wide range of signal-inputintensities isemployed herein and in the appended claims, it refers tosuch intensity variations of the received carrier as are due to fading,to the difierent field strengths of different signals, and the like, andare independent of light-modulation components, and not to the normallysmaller average carrier intensity variations which are caused byvariation of the averagebackground illumination.

It will be appreciated that the present invention may be utilizedautomatically to control various characteristics of a televisionreceiver other than its amplification, for example, its selectivity orits tuning. 1

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. A television receiver adapted for the reception ofpositively-modulated television signals including background components,synchronizing components, and video-frequency lightmodulationcomponents, comprising asynchronous control means responsive to apredetermined intermediate amplitude level of the modulation envelope ofsaid signals and including a time-constant circuit for substantiallyeliminating the-effect of said synchronizing components on the outputthereof for developing a controlling effect, and means for utilizingsaid controlling effect for adjusting an operating characteristic of thereceiver.

2. A television receiver adapted for the reception ofpositively-modulated television signals including background components,synchronizing pulses of a given type, and video-frequencylight-modulation components, comprising asynchronous control meansresponsive to a prea predetermined determined intermediate amplitudelevel of the modulation envelope of said signals occurring for durationgreater than the duration of one of said synchronizing pulses and duringany portion thereof for developing a controlling effect, and means forutilizing said controlling effect for adjusting an operatingcharacteristic of the receiver.

3. A television receiver adapted for the reception ofpositively-modulated television signals including background components,synchronizing components, and video-frequency light-modulationcomponents and having a predetermined blanking level, comprisingasynchronous control means responsive solely to the blanking level ofthe modulation envelope of said signals and including a time-constantcircuit for substantially eliminating the effect of said synchronizingcomponents on the output thereof for developing a controlling effect,and means for utilizing said controlling effect for adjusting anoperating characteristic of the receiver.

4. A television receiver adapted for the reception ofpositively-modulated television signals including background components,synchronizing components, and video-frequency light-modulationcomponents and having a wide range of input intensities, comprisingasynchronous control means responsive to a predetermined intermediateamplitude level of the modulation envelope of said signal and includinga time-constan't circuit for substantially eliminating the effect ofsaid synchronizing components on the output thereof for developing acontrolling effect, and means for utilizing said controlling effect tomaintain the signal-output intensity of the receiver within a relativelynarrow range.

5. A television receiver adapted for the reception ofpositively-modulated television signals including background components,synchronizing components, and video-frequency light-modulationcomponents having a predetermined blanking level occurring duringretrace periods and having a wide range of signal-input intensities,comprising a signal-amplifying channel, and asynchronous control meansresponsive to the blanking level of the modulation envelope of saidsignals and including a time-constant circuit for substantiallyeliminating the effect of said synchronizing components on the outputthereof for developing a controlling effect, and means for utilizingsaid controlling effect to maintain the signal-output intensity of saidchannel within a relatively narrow range.

6. A television receiver adapted for the reception of a televisionsignal carrier positively modulated during trace periods by backgroundand video-frequency light components and umnodulat-ed by lightcomponents during retrace periods and modulated by synchronizing pulsesoccurring during said retrace periods, comprising means for deriving themodulation envelope of said signal with the peaks of said synchronizingpulses thereof at a fixed level, asynchronous control means fordeveloping a controlling effect from said modulation envelope variablein accordance with variations in a predetermined intermediate amplitudelevel thereof and including a time-constant circuit for substantiallyeliminating the effect of said synchronizing pulses on the outputthereof, and means for utilizing said controlling effect to adjust anoperating characteristic of the receiver.

'7. A television receiver adapted for the reception of a televisionsignal carrier positively modulated during trace periods by backgroundand retrace periods, comprising means for detecting video-frequencylight components and unmodusaid signal to derive the modulationenvelope, a

lated bylight components during retrace periods vacuum tube forstabilizing said detected sigwith synchronizing pulses occurring duringsaid nals having an input circuit. non-conductively retrace periods,comprising means for detecting 5 coupled to said detecting means and anoutput said signal to derive the modulation envelope, circuit having apredetermined time constant for means for stabilizing the detectedsignal to essubstantially eliminating the eifect of said syntablish afixed level for the peaks ,of said synchronizing pulses to develop anasynchronous chronizing pulses, asynchronous control means controlsignal from said stabilized detected sigfor developing a controllingeffect from said stal0 nal variable in accordance with variations in a.bilized detected signal variable in accordance predeterminedintermediate amplitude level with variations in a predeterminedintermediate thereof, a peak rectifier coupled to said output amplitudelevel thereof and including a timecircuit for developinga'unidirectional-bias voltconstant circuit for substantially eliminatingthe age variable in accordance with variations in the eflect of saidsynchronizing pulses on the output peak amplitude of said controlsignal, and means thereof, and means for utilizing said controlling forutilizing said bias voltage to adjust an opereffect to adjust anoperating characteristic of ating characteristic of the receiver.

the receiver. 10. A television receiver adapted for the re- 8. Atelevision receiver adapted for the recepception of a television signalcarrier positively tion of a television signal carrier positivelymodmodulated during trace periods by background ulated during traceperiods by background and and video-frequency light components andunvideorequency light components and unmodumodulated bylight componentsduring line and lated by light components during retrace periods fieldretrace periods and having a predetermined and having a predeterminedblanking level with blanking level with synchronizing pulsesoccursynchronizing pulses occurring during said rering during saidretrace periods, comprising trace periods, comprising means fordetecting means for detecting said signal to derive the said signal toderive the modulation envelope, modulation envelope, a vacuum tube forstabilizmeans for stabilizing the detected signal to esing said detectedsignals having an input circuit tablish a fixed level for the peaks ofsaid synnon-conductively coupled to said detecting means chronizingpulses, asynchronous control means and an output circuit having apredetermined for developing a controlling eflect from said statimeconstant greater than the duration of said bilized detected signalvariable in accordance synchronizing pulses but less than the normalwith variations in the blanking level and induration of said fieldretrace periods for developcludlng a time-constant circuit forsubstantially ing an asynchronous control signal from said eliminatingthe effect of said synchronizing stabilized detected signal variable inaccordance pulses on the output thereof, and means for utiwithvariations in the blanking level thereof, a lizing said controllingefiect to adjust an operpeak rectifier coupled to said output circuitfor ating characteristic of the receiver. developing aunidirectional-bias variable voltage 9. A television receiver adaptedfor the recepin accordance with variations in the peak amplition of atelevision signal carrier positively modutude of said control signal,and means for utilated during trace periods by background and lizingsaid bias voltage to adjust an operating vid -irequency light componentsand unmoducharacteristic of the receiver. lated by light componentsduring retrace periods 1 with synchronizing pulses occurring during said1 JOHN C. WILSON.

